The present invention relates to a method for determining the position of a receiver using signals transmitted, e.g. by a constellation of satellites. The receiver is typically located at low altitudes. In particular, the receiver can be located in an aircraft for navigating during final approach and even for landing the aircraft, in a ship or a vehicle moving over ground. More particularly, the present invention relates to a reduced-noise ionosphere-free smoothing of ranging codes.
Satellite navigation is part of nowadays daily life and is used for an increased number of applications. The signals transmitted by the satellites and received by receivers comprise at least one ranging code modulated on at least one carrier. The ranging code is a sequence of binary values, called chips. The rate of the ranging code is of the order of a few Mchips/s, and the frequency of the carrier signal is of the order of GHz. Under ideal conditions, the ranging code is suitable for determining the code phase with an accuracy of the order of 1/1000 of a chip. The ranging code is furthermore modulated by information that allows determining the time of transmission. The combination of the two pieces of information is the basis for the estimation of the distance between the transmitter and the receiver.
Real signals are delayed by an unknown delay in the ionosphere, and are corrupted by multipath propagation to name but a few. The code phase, i.e., the delay determined by the ranging code is rather affected by multipath and is also rather noisy. Both are much less the case for the carrier phase. The carrier phase is the phase of the periodic carrier oscillation. It repeats after one wavelength, which often is of an order of a few centimeters. “Smoothing” using a Hatch filter is a known method which takes advantage of the less affected ambiguous carrier signal for improving the quality of the code phase estimation. The traditional Hatch filter subtracts the carrier phase from the code phase, filters the so obtained signal, and then restores the carrier contribution.
In this connections carrier smoothing is one of the known methods for reducing noise variance. Carrier smoothing is used today for safety critical applications, like navigation of aircraft during flight and early final approach.
The problem in satellite navigation is that due to disturbance in the ionosphere unexpected and unforeseeable stochastic delays occur. In order to eliminate ionosphere induced errors, the use of code combinations and carrier combinations n the smoothing are desirable. All code combinations considered so far have, however, increased the noise variance. Due to this increased noise variance, the known carrier smoothing concepts cannot be used for high security applications like navigation of an aircraft during late final approach and landing phase.
Accordingly, what is needed is a reduced-noise ionosphere-free carrier smoothing with increased performances.